Geoscience Reference
In-Depth Information
Charge-
Coupled
Device
Monochromator
{Poly/Triple/Double}
Aqueous
Sample
Cuvette
Laser
Figure 5.5. Basic configuration of a Raman spectrometer.
one can look at backscattered Raman, for example, in the Raman microscope. In Raman
spectroscopy, there are several important factors to consider when designing an experimen-
tal system. Because the analyte scatters light with a frequency shifted with respect to the
input light, the excitation source is almost always a monochromatic laser. Also, because
Raman scattering is a relatively weak phenomenon, it is necessary to avoid simultaneous
detection of the scattered light and of the input laser (incident light), which would dwarf
the desired scattered signal.
Figure 5.5
demonstrates a typical configuration of a Raman
spectrometer for liquid samples. It is important to understand that polarization of the input
and output is important, such as vertical polarization (VV) or where the input and output
exhibit vertical and horizontal polarization (VH) respectively.
Also competing with Raman scattering is the much more intense Rayleigh scattering.
Because Raman and Rayleigh scattering are of different wavelengths, Rayleigh scattered
light can be filtered at relatively little expense to the Raman signal using a notch filter or
a double or triple monochromator. For some materials, their highly fluorescent properties
may dominate the Raman scattering. In such cases the use of longer wavelength lasers (to
a certain extent), time-resolved techniques, and fast Fourier transform Raman eliminate
this problem. Techniques such as surface-enhanced Raman scattering, resonance Raman
scattering, and coherent anti-Stokes Raman scattering greatly enhance the signal with
respect to spontaneous Raman scattering. One area where Raman spectroscopy is particu-
larly useful is in the determination of species in aqueous media. In these solutions, water
is the dominant species with the molecules of interest existing at lower concentrations.
In this scenario infrared absorption may be ineffective because water absorption may
dominate the spectrum and obscure the desired results. Water molecules exhibit intrinsi-
cally weak Raman signals; even so, Raman spectroscopy provides a useful tool for solute
determination.
